A polymer electrolyte fuel cell (PEFC) is a fuel cell with the form in which a solid polymer electrolyte is sandwiched between an anode and a cathode, a fuel is supplied to the anode, oxygen or air is supplied to the cathode, oxygen is reduced at the cathode to produce electricity. As the fuel, hydrogen, methanol, or the like is mainly used. To enhance the reaction rate in a fuel cell and to enhance the energy conversion efficiency of the fuel cell, a layer containing a catalyst has been conventionally disposed on the cathode and anode surfaces of the fuel cell. As the catalyst, noble metals have been generally used, and, among the noble metals, platinum, which is stable at a high potential and has a high activity, has been mainly used. As a carrier on which the catalytic metal is carried, carbon black has been conventionally used.
In the PEFC, the cathode is temporarily exposed to a high potential, for example, around 1.5 V during repeated start/stop operation. It is known that under such a high potential, carbon black which is a carrier is oxidatively corroded in the presence of water, a catalytic metal falls or aggregates, the deterioration of catalytic activity, the deterioration of the conductivity of a catalyst layer, or the like occurs, and electricity generation performance is deteriorated. Therefore, there have been demanded a carrier or a catalyst having durability to a high potential caused by starting and stopping; and a fuel cell electrode catalyst in which the carrier or the catalyst is used.
Against such a problem, it has been examined to suppress carbon corrosion of a catalyst carrier in start/stop of PEFC.
Patent Literature 1 describes an electrode catalyst obtained by heat treatment of a carbon material, on which a noble metal catalyst is carried, under inert gas atmosphere. The literature discloses that an amorphous portion on the surface of the carbon material is removed utilizing the catalytic action of a noble metal catalyst (for example, Pt or Pt alloy), the graphitization degree of a carrier surface is improved, and the durability to carbon corrosion can be further improved. The graphitization degree is defined as a ratio (I1355/I1580) of intensity (I1355) at Δν1355 (peak in the vicinity of 1355 cm−1 in Raman spectrum) to intensity (I1580) at Δν1580 (peak in the vicinity of 1580 cm−1 in Raman spectrum) in a Raman spectrum. It is described that the graphitization degree is more improved with decreasing the value of the ratio.
Patent Literature 2 describes a carrier for carrying a catalyst, obtained by carbonizing a raw material containing a nitrogen-containing organic substance and a metal. In addition, the carrier may be a carrier for carrying a catalyst, having an intensity ratio (I1360/I1580) of a band at 1360 cm−1 to a band at 1580 cm−1 in a Raman spectrum of 0.3 or more and 1.0 or less. The carrier is included in an appropriate balance indicated by the range of the intensity ratio D/G (I1360/I1580) as described above, to be able to be thereby provided with both of high durability and high catalyst-carrying performance.
Patent Literature 3 describes a catalyst comprising a metal element M, carbon, nitrogen, and oxygen, wherein peaks are observed at 1340 cm−1 to 1365 cm−1 and at 1580 cm−1 to 1610 cm−1 as measured by Raman spectroscopy, and the metal element M is one selected from the group consisting of titanium, iron, niobium, zirconium, and tantalum. It is described that the catalyst which is not corroded in an acidic electrolyte or at a high potential is stable. It is described that assuming that the height of the peak at 1340 cm−1 to 1365 cm−1 is D while the height of the peak at 1580 cm−1 to 1610 cm−1 is G (with the proviso that D and G are the heights obtained by subtraction of a baseline height), D/G is preferably 0.1 or more and 10 or less, and D/G of 0.1 or more and 10 or less is considered to allow electrons to be supplied to a site having a high activity and is desirable for an electrode catalyst.
Patent Literature 4 describes a composite material of nanostructured graphite, comprising a graphite aggregate in which primary particles of nanostructured graphite in which the sizes of crystallites are 1 to 20 nm aggregate, wherein either metal or metal oxide is allowed to be contained in, carried by, or compounded into the nanostructured graphite in which the average particle diameter of the graphite aggregate is 0.5 to 50 μm. The intensity ratio (I1360/I1580) between the Raman bands of the nanostructured graphite may be in a range of 0.4 to 1.7. Limitation into the range achieves a carbon material with improvement in conductivity and with a structure optimal for a Pt fine particle-carried material.